This invention relates to processing of oxide superconductor composites to obtain high density, textured oxide superconductor articles.
Polycrystalline, randomly oriented oxide superconductor materials are generally characterized by their low density and low critical current densities. High oxide density, good oxide grain alignment and grain interconnectivity, however, are associated with superior superconducting properties.
Composites of superconducting materials and metals are often used to obtain better mechanical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by the well-known xe2x80x9cpowder-in-tubexe2x80x9d or xe2x80x9cPITxe2x80x9d method. When powders include metal oxides or other oxidized metal salts, the method is referred to as xe2x80x9coxide-powder-in-tubexe2x80x9d or OPIT. For multifilamentary articles, the method generally includes the three stages of (a) forming a powder of superconducting precursor materials (precursor powder formation stage), (b) filling a noble metal billet with the precursor powder, longitudinally deforming and annealing it, forming a bundle of billets or of previously formed bundles, and longitudinally deforming and annealing the bundle to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material surrounded by a noble metal matrix (composite forming stage); and (c) subjecting the composite to successive asymmetric deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing stage). General information about the OPIT method described above and processing of the oxide superconductors is provided by Sandhage et al. in JOM, Vol. 43, No. 3 (1991), pp 21-25, and references cited therein; by Tenbrink et al., xe2x80x9cDevelopment of Technical High-Tc Superconductor Wires and Tapesxe2x80x9d, Paper MF-1, Applied Superconductivity Conference, Chicago (Aug. 23-28, 1992); and by Motowidlo et al., xe2x80x9cProperties of BSCCO Multifilament Tape Conductorsxe2x80x9d, Materials Research Society Meeting, Apr. 12-15, 1993, all of which are incorporated by reference.
The deformations of the thermomechanical processing state are asymmetric deformations, such as rolling and pressing, which create alignment of precursor grains in the core (xe2x80x9ctexturedxe2x80x9d grains) and facilitate the growth of well-aligned and sintered grains of the desired oxide superconducting material during the later thermal processing stages. A series of heat treatments is typically performed during the thermomechanical processing stage to promote powder reactions, including the final thermomechanical processing stages employed to fully convert the filaments to the desired highly textured superconducting phase.
In the practice of the above prior art approach, it has been found that when heating during the thermomechanical processing stage, the oxide grains experience dilation leading to reduced oxide core density and increased porosity of the oxide core. Dilation is the loss of core material density due to introduction of pore space and/or changes in grain size and structure. Dilation is thought to be caused by gas evolution and by the growth of non-aligned oxide grains during heating.
Achieving high density in ceramics and ceramic composites is not a new problem. For other ceramic systems, such as Al2O3 for structural problems, high density is achieved by heating the final product under high pressure.
Current approaches to rectifying the de-densification arising from the annealing process include mechanical deformation to redensify the oxide material. For example, Dou et al., in xe2x80x9cImprovements of Critical Current Density in the Bixe2x80x94Pbxe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O System Through Hot Isostatic Pressingxe2x80x9d (Physica C, 167:525 (1990)), report similar results by hot isostatically pressing (HIPing) BSCCO pellets and powders. Bourdillon et al., in xe2x80x9cHot Isostatically Pressed Bi2Sr2Ca2Cu3O10 Coils Made with Novel Precursors,xe2x80x9d describe HIPing of a BSCCO 2223 coil. Nhien et al., in xe2x80x9cBulk Texturing of Prereacted Bi/Pb(2223) under Triaxial Stresses at Room Temperaturexe2x80x9d (Physica C 235-240:3404 (1994)), use isostatic confinement coupled with an overload in one direction to promote grain alignment of a fully formed (Bi,Pb)SCCO 2223 material. International Application Publication No. WO 94/00886, entitled xe2x80x9cHigh Tc Superconductor and Method of Makingxe2x80x9d and published Jan. 6, 1994, also describes an isostatic pressing operation after a heat treatment to impart superconducting properties to the precursor and before a final heat treatment to complete the phase conversion.
These approaches represent attempts to modify the oxide grain structure after de-densification of the oxide core has occurred. Such deformation steps are carried out when phase conversion to the final desired oxide superconductor is complete or nearly complete. While deformation processing may result in increased core density, at this late stage in the process it introduces both intergranular and intragranular cracks in the oxide phase that are highly resistant to healing by conventional annealing processes.
Other examples in the prior art use HIPing to introduce texture into the oxide superconductor composite, in particular, in those instances where asymmetric deformation is not preferred. EP 0 503 525 discloses the preparation of a twisted, multifilamentary oxide superconductor composite. The method relies upon drawing to alter the cross-sectional size and shape of the filaments during assembly of the multifilamentary composite, a method that is known to be ineffective in producing a high density, highly textured oxide phase, i.e., such methods result in f less than 0.7 (as defined herein) and density within the filaments is less than 60% theoretical. In order to texture to a degree approaching acceptable levels, the composite is HIPed. Thus, HIPing is used to introduce density or texture into the composite and not to retain any previously introduced texture of the composite during subsequent processing steps. EP 0 503 525 does not address the problem of dilation, since the oxide phase was not significantly densified and textured in the first place.
Furthermore, not all deformation processes have the desired effect of texturing and/or densifying the oxide material. See Pachla et al., in xe2x80x9cThick textured films of Bi-type ceramics by hot pressingxe2x80x9d (Applied Superconductivity, 1(1-3):745 (1993)), who report hot pressing of BSCCO 2212 phase. The process resulted in a significant crushing of the oxide superconductor phase, and did not show evidence of texturing or densification of the BSCCO 2212 phase.
Still other groups have used HIPing to densify powder compacts of the oxide superconductor. Tien et al., in xe2x80x9cDensification of Oxide Superconductors by Hot Isostatic Pressingxe2x80x9d (Metallur. Trans. A, 19A:1841 (July 1988)), report an increase in density of a YBa2Cu3Ox powder compact from 65% theoretical density to 99% of theoretical. HIPing was performed on a fully formed oxide superconductor.
Thus, there remains a need to overcome the problem of dilation, i.e., de-densification, of the oxide core material in multifilamentary composites during heat treatments, without the drawback of introducing cracks or other defects in the process. What is needed is a process that prevents or substantially prevents dilation from occurring in the first instance. Such a process would clearly present great advantages over the prior art processes in that no remedial action is required.
The present invention overcomes the limitations of the prior art by converting a highly textured oxide superconducting precursor into an oxide superconductor, while simultaneously applying a force to the oxide superconductor precursor which at least matches the expansion force experienced by the precursor during phase conversion to the oxide superconductor, whereby the density and the degree of texture of the oxide superconductor precursor are retained or substantially retained during phase conversion.
In one aspect of the invention, a method of making an oxide superconductor article includes converting a textured oxide superconducting precursor into a selected oxide superconductor, while simultaneously applying a force to the precursor which at least matches the expansion force experienced by the precursor during phase conversion to the selected oxide superconductor, whereby the near net shape of the oxide superconductor precursor is substantially retained during phase conversion.
By xe2x80x9chighly texturedxe2x80x9d as that term is used herein, it is meant that the oxide grains have been oriented and aligned to a significant degree. Most commonly, the extent of texture development in an oxide material is quantified using an f-factor. A lotgering factor, or f-factor, of a material is obtained from the X-ray diffraction (XRD) pattern of the material by relating the peaks associated with the oriented grains to those of all peaks. In the case of c-axis aligned BSCCO-2212, the 001 intensities are an indication of texture. When the oxide is fully aligned, no other peaks are observed in the XRD pattern and the ratio, i.e., f-factor, is 1.0. When the oxide is fully random, the f-factor is zero. In preferred embodiments, the f-factor has a value of at least 0.7, more preferably at least 0.8 and most preferably at least 0.9. In the preferred processing methods described herein to obtain a highly textured oxide, f-factors on the order of 0.95 are attainable.
By xe2x80x9chighly densexe2x80x9d as that term is used herein, it is meant a density that is at least 70% dense relative to theoretical. Preferably, highly dense oxides possess a density that is greater than 80%, and most preferably greater than 90% dense, relative to theoretical (in which the material has no pore space or voids).
By xe2x80x9cnear net shapexe2x80x9d as that term is used herein, it is meant the external physical dimensions of the filamentary article. Because the length of the article is not greatly affected by the processing described herein, the near net shape is defined by the cross-sectional shape of the article as measured by the external perimeter of the article cross-section. The parameter correlates closely to dilation of the article, in that, greater dilation during processing results in greater change in near net shape. Changes in near net shape are determined with reference to the cross-sectional shape of the article as measured by the external perimeter of the article cross-section before and after phase converting heat treatments. In preferred embodiments, articles of the invention demonstrate overall changes in near net shape of less than 1% as a result of phase conversion, i.e., the external physical dimensions of the article remain substantially unchanged. Minor changes in near net shape are highly advantageous as it allows one to set the dimensions of the final product early on in the process in forming the precursor.
By xe2x80x9coxide superconductor precursor,xe2x80x9d is meant any material that can be converted into the selected and final oxide superconductor under appropriate conditions, e.g., a suitable heat treatment. Suitable precursor materials include, but are not limited to, metal salts, simple metal oxides, complex mixed metal oxides, and even an intermediate oxide superconductor. An intermediate oxide superconductor is an oxide superconductor, which is capable of being converted into a selected oxide superconductor. It often is used as a mixed phase material in combination with other components, typically non-superconductive, which in combination can be reacted to form the final oxide superconductor. An intermediate oxide superconductor may have desirable processing properties, which warrants its formation initially before final conversion. For example, an intermediate oxide superconductor may be more amenable to texturing than the final oxide superconductor.
The phase conversion may involve a change in composition or a change in crystallographic phase.
In most instances, the precursor material and final oxide superconductor differ in composition. That is, the compounds defining the precursor are different than those of the selected oxide superconductor. For example, in the preparation of BSCCO 2223, the precursor typically includes an intermediate BSCCO 2212 oxide superconductor and secondary metal oxides.
In other instances, the precursor material and the final oxide superconductor may have the same composition, but may differ in crystallographic phase. By crystallographic phase is meant that the lattice symmetry differs such that the crystallographic characterization of the material differs. Examples of materials that differ crystallographically are different symmetry phases of materials having the same or similar composition, such as the tetrahedral and orthogonal phases of BSCCO 2212.
In some embodiments, the oxide superconductor precursor is subjected to a texturing operation to orient grains of the oxide superconductor precursor to obtain the highly textured precursor. According to the invention, the article is textured prior to phase conversion, and the phase-converting heat treatment is not relied upon to also texture the article.
In other embodiments, the oxide superconductor precursor and the selected oxide superconductor differ in composition, or in another embodiment, they differ crystallographically.
In other embodiments, the density of the oxide superconductor precursor is substantially retained during phase conversion. In alternative embodiments, the texture of the oxide superconductor precursor is substantially retained during phase conversion.
In some embodiments, the near net shape changes less than 7%, preferably less than 6%, preferably less than 3%, and more preferably less than 1.5% between the precursor and the final article.
In other embodiments, substantially retained includes a change of less than 20%, preferably less than 10%, and more preferably less then 5% in density between the precursor and the final article.
In still other embodiments, substantially retained includes a change of less than 10%, preferably less than 5%, and more preferably less than 2.5% in degree of texture between the precursor and the final article.
In some embodiments, the force applied to the precursor comprises hot isostatic pressing (HIPing), and the HIPing force is in the range of 10 to 2500 atm (1-250 MPa), and preferably in the range of 25 to 250 atm (2.5-25 MPa).
In other embodiments, dilation of the oxide superconductor article is less than 7%, and preferably less than 6% in the direction transverse to the direction of elongation, and more preferably is in the range of 1-6% in the direction transverse to the direction of elongation.
In other embodiments, the precursor oxide comprises BSCCO 2212, and the final oxide superconductor comprises BSCCO 2223, and preferably, the precursor oxide comprises BSCCO 2212, and the final oxide superconductor comprises BSCCO 2223, and wherein the temperature and oxygen partial pressure are selected to fall in a range in which BSCCO 2223 is thermodynamically stable.
In still other embodiments, the pressure is constant during phase conversion.
In still other embodiments, the highly textured precursor possesses a degree of texture greater than or equal to 0.7, and preferably greater than 0.8, and more preferably greater than 0.9.
In alternative embodiments, the precursor is textured using asymmetric deformation, and preferably the asymmetric deformation is selected from the group consisting of rolling and pressing. The rolling deformation results in a 40-95% reduction in thickness of the article.
In another embodiment, the precursor is textured using reaction-induced texturing, and preferably, the precursor comprises BSCCO 2212 and reaction induced texturing is conducted at a temperature in the range of 800-860xc2x0 C. and an oxygen partial pressure in the range of 0.01-1.0 atm (1-100 xc3x9710xe2x88x923 MPa).
In another embodiment, BSCCO 2212 is converted into BSCCO 2223 in a two-step heat treatment in which the precursor is heated under conditions which form a liquid phase in co-existence with BSCCO 2223 and then the precursor is heated under conditions which transform the liquid phase into BSCCO 2223.
In another aspect of the invention, an oxide superconductor composite includes one or more filaments of an oxide superconductor in a metal matrix, wherein the oxide superconductor composite demonstrates a near net shape change of less than 6% between a precursor to the oxide superconductor composite and the oxide superconductor composite.
In some embodiments, the oxide superconductor composite demonstrates a near net shape change after processing of less than 3% and preferably less than 1.5% between the precursor and the oxide superconductor composite.
In another aspect of the invention, a simplified process for making an oxide superconductor article of acceptable texture, density and current carrying properties is provided. The process includes providing a precursor composite comprising one or more filaments of BSCCO 2212 in a metal matrix, texturing the BSCCO 2212 precursor composite in a rolling deformation step, wherein the rolling deformation results in a 40-95% reduction in thickness of the composite, converting the textured BSCCO oxide superconducting precursor into a BSCCO 2223 oxide superconductor, while simultaneously applying a force to the precursor which at least matches the expansion force experienced by the precursor during phase conversion to the BSCCO 2223 oxide superconductor, whereby the near net shape of the oxide superconductor precursor is substantially retained during phase conversion and whereby no further densification or texturing of the composite occurs.
In another aspect of the invention, a BSCCO 2223 oxide superconductor composite includes one or more filaments of a BSCCO 2223 oxide superconductor in a metal matrix, wherein the oxide superconductor has a density of at least 90% theoretical density and an f-factor of at least 0.7, preferably at least 0.8, and more preferably at least 0.9. The composite is substantially free of cracks and defects as observed under an optical microscope.
An advantage of the method of the invention is the elimination of costly processing steps without detrimental effect on the superconducting properties of the article. Thus, the process may be reduced to a single deformation and heat treatment (a xe2x80x9c1DSxe2x80x9d process) because there is no need to carry out an intermediate deformation (and subsequent heat treatment) to redensify the material.